1. Technical Field
The present invention relates to a device for reducing at least one gas component in an exhaust gas flow of a combustion engine. The engine is adapted for operation on a lean air/fuel mixture and includes an exhaust pipe for transport of the exhaust gas flow from the engine. The invention is particularly intended for reduction of harmful emissions from the exhaust gas flow; and more particularly, via a method and arrangement incorporating a separation unit for this purpose.
2. Background of the Invention
In the field of vehicles that are operated by combustion engines, there is a general demand for low emissions of harmful substances in the exhaust gases from the engine. These substances are primarily constituted by pollutants in the form of oxides of nitrogen (NOx), hydrocarbon compounds (HC), and carbon monoxide (CO). Regards conventional petrol engines, the exhaust gases are typically purified by means of an exhaust catalyst that forms part of the exhaust system and through which the exhaust gases are guided. In a so-called three-way catalyst, the major part of the above-mentioned harmful compounds is eliminated by means of conventional catalytic reactions. In order to optimize the function of the catalyst so that it provides an optimal degree of purification for NOx HC, and CO, the engine is in most operating conditions operated by a stoichiometric air/fuel mixture; that is, a mixture where lambda=1.
Furthermore, in today's vehicular environment, there is a general demand for reducing the fuel consumption of the engine to the highest possible degree. To this end, during the last few years, engines with new types of combustion chambers in the engine's cylinders have been developed; in particular in ways that enable the engine to operate by increasingly lean fuel mixtures, i.e. where lambda=1. In such an engine, which normally is called a “lean-burn” engine, or alternatively a “Dl engine” direct-injected Otto cycle engine), the respective combustion chamber in the engine is arranged in such manner that the supplied fuel to a great degree can be concentrated at the respective ignition plug. This mode of operation is generally termed “stratified” operation and during continuous driving at a low or a medium-high torque and engine speed of the engine, it can operate on a very lean air/fuel mixture; more precisely, up to approximately lambda=3. In this manner, a substantial savings in fuel consumption is obtained using this type of engine. The engine can also be operated in an additional, “homogeneous” mode of operation with an essentially stoichiometric mixture (lambda=1) or a comparatively rich mixture (lambda=1). This latter mode of operation normally prevails during driving situations with comparatively high torques and speeds of the engine.
During stratified operation, a lean exhaust gas mixture will flow through the three-way catalyst. This corresponds to an exhaust gas mixture with a surplus of oxygen in relation to what is the case during lambda=1. This results in the three-way catalyst not being properly utilized for reduction of the NOx compounds in the exhaust gases due to the fact that it is constructed for an optimal purification capacity during stoichiometric mixture. In such cases, there becomes a demand for additional devices and methods for the reduction of NOx compounds. This demand also arises in other types of engines that are operated by a surplus of oxygen and where NOx compounds are generated during operation, which, for example, is the case with diesel engines.
In order to provide a reduction of NOx compounds from a “lean-burn engine” the engine can be provided with a nitrogen oxide adsorbent (also-called NOx adsorbent, or “NOx trap” for absorption of NOx compounds in the exhaust gases from a combustion engine. The NOx adsorbent can be utilized as a complement to a conventional three-way catalyst either as a separate unit upstream of the three-way catalyst or as an integral part of the three-way catalyst; that is, together with the catalytic material of the three-way catalyst.
The NOx adsorbent is constructed in such manner that it takes up (adsorbs) NOx compounds from the exhaust gases if the engine is operated by a lean air/fuel mixture and gives off (desorbs) the NOx compounds if the engine is operated by a rich air/fuel mixture during a certain time period. Furthermore, the NOx adsorbent has the property of being able to adsorb NOx compounds only up to a certain limit; that is, it is eventually “filled” and thus reaches a limit for the adsorption. In this situation, the NOx adsorbent must be regenerated; that is to say, it must be influenced or caused to desorb, and thus release the accumulated NOx compounds. If a conventional three-way catalyst is provided downstream of a NOx adsorbent, or if alternatively a three-way catalyst is formed as an integral part of at NOx adsorbent, provided that the latter has reached its ignition temperature.
A NOx adsorbent can typically be regenerated by means of the fact that the exhaust gas mixture that flows through the NOx adsorbent is made comparatively rich during certain time periods usually extending over approximately a few seconds. In practice, this is achieved by the fact that the engine, during this time period, is operated in the above-mentioned homogeneous mode of operation, wherein the engine is operated on a comparatively rich air/fuel mixture. By means of this “rich pulse” a surplus of CO and H2 molecules is generated that functions as a reduction agent which in turn reacts with NOx compounds according to the following:Ox+R−>N2+CO2+H2O                where R schematically indicates the relevant reduction agent. In this manner, a large part of the NOx compounds in the exhaust gas flow can be eliminated by means of transformation into molecular nitrogen, carbon dioxide and water. Then, the engine once again can be reset to lean operation, by means of which the NOx adsorbent absorbs NOx compounds during a certain time period that lasts until a new regeneration becomes necessary.        
Thus, according to what has been described above, the NOx compounds are reduced by means of a reduction agent which is taken from the engine's own fuel; that is, it is generated in the engine during the short time period during which the engine is operated during rich conditions. Furthermore, a control unit is utilized with a suitable strategy for switching the combustion engine between homogeneous and stratified operation depending on whether a NOx regeneration is necessary and depending on the engine's mode of operation in other respects such as depending on the relevant degree of throttle application and the engine speed.
Although the above-mentioned course of events for regeneration of a NOx adsorbent in principle functions satisfactorily, it suffers from certain drawbacks. For example, it can be noted that it is necessary to operate the engine by a rich exhaust gas mixture for regeneration of the NOx adsorbent, and this requires an accurate control of the engine's mode of operation, particularly for switching between rich and lean operation, respectively. Moreover, the regeneration results in a surplus of fuel being supplied to the engine during the rich pulse. This in turn causes the engine's fuel consumption to be influenced in a negative way.
An additional drawback associated with the regeneration of a NOx adsorbent by means of supply of a reduction agent is that a large part of the reduction agent reacts with the oxygen molecules that are present in the exhaust gas flow. Thus, the HC, H2 or CO molecules in the exhaust gas flow that could react with NOx compounds in the exhaust gases and form harmless N2 instead, to a great extent, react with oxygen molecules in the exhaust gases and this impairs the efficiency of the process.
Another known manner of reducing NOx compounds is to supply a reduction agent in the form of ammonia/urea (so-called SCR technique) to the relevant gas flow. A drawback of this method, however, is that it requires special arrangements for storage and supply of ammonia/urea, and that a NOx reduction by means of this technique only is allowed within a certain temperature interval; more precisely, approximately 300-5000°.
Still another known manner for reducing NOx compounds in engine exhaust gases is to utilize a so-called EGR system (Exhaust Gas Recirculation), wherein a certain amount of the exhaust gases from the engine is returned to the inlet of the engine.
Yet another known manner of reducing NOx compounds in engine exhaust gases is to utilize zeolite structures with pores of two different sizes (so-called “dual pore size” technique). In this case, NOx molecules, for example, in a gas flow which passes the zeolite structure will be transformed into NO2 molecules in the pores of the smaller size whereupon the NO2 molecules react with a reduction agent such as an HC compound in the pores of the larger size. In this case, by means of the last-mentioned reaction, N2, CO2 and H2O are formed.
Other HC-based systems can also be utilized for NOx reduction such as in a system that is based on an oxide of aluminum (Al2O3) to which silver atoms have been supplied. In such a structure, an HC compound can be supplied at the same time as a gas flow containing NOx compounds is allowed to pass the structure. This results in a decrease of the NOx compounds in the gas flow.